The remote sensing of atmospheric trace gases is of great importance. In particular, obtaining accurate measurements of atmospheric trace gas species, such as CO and CO2, from an aircraft or spacecraft platform is essential for improving the scientific understanding of global atmospheric chemistry, climate impacts, and the atmospheric component of the global carbon budget.
One way of obtaining information regarding the amount of atmospheric trace gases is to sense the spectral absorption of reflected sunlight. In particular, the amount of absorption of light at wavelengths corresponding to the spectral lines of the gas of interest can be detected and measured. In general, the higher the absorption of light at such wavelengths, the higher the concentration of the associated gas in the portion of the atmosphere from which the sampled light was collected. Similarly, the absorption of thermal emissions by atmospheric trace gases can be measured to obtain information regarding the amount of such gases.
Various spectrometers have been developed for enabling such measurements. For example, Fourier transform spectrometers have been developed that are capable of high spectral resolution. However, such instruments are relatively large and complex. Other instruments for sensing light within a narrow range of wavelengths include devices utilizing optical cavities, such as Fabry-Perot interferometers and multiple cavity filters formed from thin films. Although optical cavity-based instruments are capable of providing high filter resolution and can be precisely tuned to have a selected passband, they are limited in the number of spectral lines that can be simultaneously detected. This is because such filters have featured regularly spaced pass bands. In particular, the passbands of conventional cavity-based filters are centered at wavelengths that are equal to integer multiples of one-quarter the wavelength corresponding to the cavity's effective depth or optical thickness. However, spectral lines associated with atmospheric trace gases are not periodically spaced. Specifically, the quantum mechanical rules that determine the permitted energies for the absorption and emission of light result in spectral lines that are not equally spaced in energy or wavelength. Accordingly, optical cavity type instruments have only been capable of detecting a small number of spectral lines (for example 2 or 3) before the pass bands of the filter are no longer sufficiently correlated with the spectral lines of the gas of interest. Therefore, the sensitivity and signal to noise ratio of such devices has been limited.
One approach to providing a filter having characteristics precisely correlated to the gas being sensed is to provide a cell containing a sample of the gas of interest. By comparing the difference between the light passed through the gas-containing cell to a detector, and light received at a detector that has not been passed through the cell, information regarding the presence of that gas in the atmosphere can be obtained. Although systems using samples of the gas being sensed are capable of providing filter characteristics that are correlated to the gas being sensed, they are difficult to implement.